Sitefinity Membership Provider is designed to work in similar way as the SQL Membership Provider. It supports three modes - Hashed (default), Encrypted and Clear. The Hashed format is default one. More information here: http://www.sitefinity.com/documentation/documentationarticles/developers-guide/deep-dive/security/users/password-format.
Hashed passwords are irreversible and you cannot achieve what you want using a hashed password format. Instead you can achieve this by either using the Clear (strongly not recommended) or the Encrypted (also not a good security practice).
However the CMS allows you to have reset password functionality or retrieve. Reset if Hashed format is used and retrieve if Encrypted is used. This article explains both approaches: http://www.sitefinity.com/developer-network/knowledge-base/configuring-password-recovery-in-sitefinity.